This invention relates generally to lumber handling and processing systems and more particularly to lumber mill systems especially suited for processing small logs.
As the cost of lumber has increased and once plentiful stands of large diameter timber have decreased, it has become increasingly necessary to harvest small diameter timber and to optimize the lumber recovery from such timber. Generally, small diameter timber, sometimes referred to as "mini-logs", have a diameter within the range of about 4 to 12 inches. Because of the relatively small yield per mini-log, economics requires that each log be processed quickly to achieve a high level of production.
Several machines have been used heretofore for processing mini-logs. One machine, known as the "Chip-N-Saw," has an endless conveyor formed in part from V-shaped lugs that supportively center the logs on the conveyor. An upper surface of each log is engaged by an endless, overhead, free-floating conveyor having interlocking C-shaped lugs which also serve to center the logs on the conveyor below. This conveyor system conveys the logs through a chipping machine which chips away the outer slabs, and then through a device which cuts a longitudinal notch or guide in the bottom of the logs. The notched log is then placed on an elongate, mating bar and run through a sawing mechanism or the like.
Another machine, known as the "Beaver," uses spiked "coleman" rolls to center the logs as they are fed through two sets of chipping heads. The first set of chipping heads slabs the top and bottom surfaces of each log, while the second set slabs the sides of the log. Multiple sets of tandem coleman rolls support opposite surfaces of the logs as they are fed through the chipping heads. At least one set of rolls grips the top and bottom surfaces of each log and at least one set grips the sides of the log.
A more recent development is the use of an endless "sharpes" chain conveyor to convey the logs through a sawing mechanism. Such conveyor carries a plurality of spaced teeth upon which the logs are impaled. Prior to impaling, each log is rolled to the preferred orientation and then centered on the conveyor. An overhead "thumper" roll then impacts the top surface of the log to impale it on the conveyor, following which the log is conveyed to the sawing mechanism. The sharpes chain conveyor works reasonably well with fairly large, heavy, straight logs, although there is still much room for improvement because the chain tends to twist about its longitudinal axis and the teeth tend to twist and rock in the log when a substantial force is applied to the log, as during sawing. Any unnecessary movement of the log during sawing makes the sawing operation less accurate.
This problem is exacerbated with small, light logs, misshapen logs and logs with hard spots, such as knots, aligned with the saw-blades because such logs are more prone to unnecessary movement on the conveyor chain during sawing. In addition, the downward force exerted by the saw blades on the front end of a fairly light log may cause the back end to raise, making the sawing operation even less accurate.
With each of the foregoing machines for processing logs into lumber, it is impossible as a practical matter to saw the logs along a vertical plane near the geometrical center or longitudinal axis of the logs. This is because these machines require some conveyance means to convey and support the logs through the sawing mechanism. Consequently, the saw blade(s) cannot be adjusted laterally to cut near the geometrical center of the centered logs without interfering with the conveyance means. Moreover, the machines are inherently incapable of conveying "off-center" logs to any reliable degree through the sawing mechanism. Thus, the ability to optimize the yield from mini-logs by adjusting the lateral position of the saw blades or otherwise is greatly restricted. For example, a centered log having a minimum diameter of 6.8 inches can be optimally processed to yield only three 2.times.4 boards. However, if the same log is sawed along a vertical plane only 0.8 inch from its geometrical center, it can be processed into four 2.times.4 boards, an increased yield of 33%. This illustration of the drawbacks of the foregoing continuous conveyor systems is even more significant when it is appreciated that the graphed distribution of harvested mini-logs is a bell curve peaking at a log diameter of about 7 inches.
Conventional "end dogging" systems do have the capacity to saw mini-logs near their longitudinal axis to optimize lumber recovery. They do so by gripping the logs laterally off-center relative to the system's longitudinal axis and then conveying the laterally offset logs through a sawing mechanism. Generally, such systems include an overhead carriage which travels on tracks and supports two end dogs for gripping the opposite ends of each log.
For example, U.S. Natural Resources, Corvallis, Oreg., manufactures an end dogging system, known as the "Log Boss System/Applied Theory", which includes log scanning means for measuring the diameter of each log and transmitting the data to a microprocessor programmed to determine the desired lateral offset of each log for optimum lumber recovery. The log is supported on a laterally movable cradle controlled by the microprocessor. The cradle offsets the log an amount determined by the microprocessor, following which the end dogs pick up the log and convey it through a three headed chipping canter. The canted log is then conveyed downstream by a linebar feed system for further processing through twin or quad bandmills. Other end dogging systems use a similar end dogging arrangement to convey the offset mini-logs through a sawing mechanism, rather than a chipping canter, to optimize lumber recovery.
Despite their high yield per mini-log, end dogging systems suffer from two major drawbacks. They are notorious for significantly lower production than most continuous conveyor type systems. Generally, end dogging systems are reciprocating type systems requiring that the end dogs transport each log downstream through the saw, release the log and then recycle back upstream to pick-up the next log. The recycling time is lost time which prevents the logs from being run through the saws essentially "end to end."
Equally important, such systems apply end pressure to the logs with little or no support for the midsection of the log. Consequently, all but the largest mini-logs tend to be somewhat unstable when subject to forces applied by the sawing mechanism during sawing. It is not unusual for mini-logs to run sideways up to an inch when one saw blade hits a knot in the log, resulting in less than optimum sawing accuracy. This problem is especially acute with small mini-logs, since the resistance of an end loaded column to bending is a function of its diameter to the fourth power. Thus, small mini-logs are much more prone to deflection during sawing than larger mini-logs.
Accordingly, there is a need for an improved lumber mill system capable of processing mini-logs, as well as larger logs, for optimum lumber recovery at a high production rate.
It is therefore one object of the invention to provide a lumber mill system capable of optimizing lumber recovery at a high production rate.
A more specific object of the invention is to provide a lumber mill system as aforesaid in which lumber recovery is optimized by laterally offsetting the logs, when appropriate, from the longitudional centerline of the conveyor means on which they are transported to the sawing mechanism.
Another object of the invention is to provide a system as aforesaid wherein the logs can be processed essentially end to end through the sawing mechanism.
Yet another object of the present invention is to provide a system as foresaid in which the lumber recovery per log is optimized without lateral adjustment of the saw blades.
A further object of the invention is to provide a system as aforesaid wherein off-center logs, misshapen logs and logs with hard spots, as well as centered logs, are firmly supported during sawing to resist unnecessary movement due to forces applied by the saw blades, thereby to promote increased sawing accuracy.
Still another object of the invention is to provide a system as aforesaid wherein the logs are firmly supported during sawing without end loading of the log.
Other objects and advantages of the invention will become apparent from the drawings and following detailed description.